I dont think he necessarily regreted leaving gantz, because he had no memory on it, so he didnt know what he left, there for, couldnt regret it.

Now, you say it seems out of character for him to do something like that, but when you take into consideration that his mind was whiped of the gantz stuff, and that he was in gantz a long time, so hes witnessed a lot of stuff, stuff that may have played a major factor in him choosing to leave gantz.

Take Kei Kurono for example, his character has went through a lot of changes at the start of gantz to where he is at now, a lot of it has to do with events that took place inside of gantz, but if his memory was whiped of everything gantz showed him, would he still be the same kei, or would he be more like he was before?

sure...

You think you would simply "regret" somethink like that??? or just say: "Oh i think i made a bad choice.... Lets kill 1000 people and get back to GANTZ" ...... if that's izumi, then i no longer think he's that cool, i think he's STUPID!

but... he can be stupid if he wants to. It's his life isn't it?------ and a 1000 persons too (shinguku's people)

Okay, let's say, for the hell of it, that he reached 100 points more then once. If he did choose the super powerful weapon before he left, where is it now? And if he chose to revive someone, what happened to them? there are far too many questions you could ask in regards to this (and by far too many i mean like 3). So it's just better to assume that he left when he first reached the 100 points.